The term means "empty space". But in the cell they are manymembrane bound sacs with little or no inner structure. Plant cells have verylarge distinct vacuoles. In fact this organelles often dominates the inside ifthe plant cell crowding all other organelles toward the cell wall. The membranesurrounding the plant cell vacuole is called the tonoplast. This a veryactive, dynamic membrane.
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Plant cells use their vacuoles for transport and storing nutrients,metabolites, and waste products. In a sense, the vacuole can be regarded asequivalent to the extra cellular space of animals. The simple space-fillingfunction of the vacuole is of great importance to plants, which capture energyfrom the sun rather than move to capture food. The mechanical stability providedby the combination of a cell wall and turgor pressure allows plant cells to growto a relatively large size, so they generally occupy a much larger volume thananimal cells. However, producing large cells by filling them with cytoplasmwould be costly both on terms of maintenance and initial synthesis. The majorityif plant cells accumulate water in their vacuoles as they become larger throughtugor driven cell-wall expansion (usually in many small vacuoles that thencoalesce to form a large vacuole.
NB The large vacuole in such cells means that mature plant cells have a large ratio of surface to cytoplasmic volume. With the cytoplasm, forming a thin layer pressed against the cell wall.
In photosynthetic cells for example, chloroplasts are arranged in a thinlayer of cytoplasm at the cell periphery facilitating exchanges.
(NB It is the plant cells cytoskeleton that organizes the cytoplasm however.)
Vacuoles Can function as a Storage Organelle
Vacuoles can store many types of molecules, in particular essentialsubstances that are potentially harmful if present in bulk in the cytoplasm. Forexample in the appropriate plant vacuoles of certain specialized cells containsuch interesting products as rubber and opium. Even ordinarymolecules such as Na+ are found in these organelles. The salts add tothe osmotic activity of the vacuole therefore contributing to the turgorpressure.
Eg. Nittella sp (alga) Their tonoplasts have Na+ pumps located in the membrane which maintain low
Since the vacuole occupies a much greater volume than the cytoplasm most ofthe cellular Na+ is in the vacuole. The different permeabilityproperties of the plasma membrane and tonoplast govern the different solutecompositions of the cytoplasm and the vacuole.
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Other molecules stored in vacuoles are involved in the interactions of theplant with animals or with other plants.